The micro-ohmmeter, also called the digital micro-ohmmeter, is a digital meter that measures low-value resistors. Its basic principle is: using a DC constant current source to generate a DC voltage drop Ux on the measured resistance Rx, and then into a digital display resistance value through voltage amplification and A/D converter. In the measurement process, the "four-terminal" (current terminal, potential terminal) measurement method is used to eliminate the error caused by the lead wire and the contact resistance. The digital micro-ohmmeter has the advantages of simple operation, time saving, digital display, and low requirements for operators.
The working principle of the micro-Eurometer is measured by the four-wire method of the bridge principle (also known as the Kelvin principle abroad), that is, a constant current is applied across the resistor, and the voltage across the two ends is detected by the instrument, and then automatically calculated by the instrument. After that, the resistance value is obtained. The advantage is that the measured data is close to the true resistance of the resistor under operating conditions and eliminates the effects of the resistance of the test line itself.
The common multimeter measuring resistor generally adopts the proportional method. The measured resistance is connected in series with the standard resistor. The standard resistance and the voltage of the measured resistor are measured. The currents of the two are the same, and the resistance of the measured resistor is converted according to the resistance of the standard resistor. The actual measurement circuit also has a voltage corresponding to the standard resistance as the reference voltage, so that the voltage across the measured resistance can be directly measured.
Therefore, when measuring the micro-resistance, the milliohm meter or micro-ohm meter is more reactive with the actual resistance value. The multimeter's test line resistance affects the true value of the measured resistance. For example, to measure a 1 ohm resistor, if the test line of the multimeter has 1 ohm itself, then the displayed resistance is 2 ohms. Do you say that the resistance is true? But the milliohms and micro-European tables do not have this confusion.
The Kelvin test, a resistive or voltage measurement technique that uses separate current-carrying and voltage sensing, enables more accurate measurements than traditional two-terminal (2T) sensing. The key advantages of four-wire detection are the separate current application unit and voltage measurement unit, which eliminates the impedance of the wiring and contact resistance.
As shown in the figure below, when we need to measure the resistance R, the sampling current is applied by a separate current source and then tested by another voltage measuring unit. In fact, you can see that the impact of other resistors Rl still exists, but how do we mitigate or eliminate the impact?
For the current source, the Rl resistor and the measuring resistor are connected in series and have no effect. We can still guarantee the current applied to the current source through the current of the resistor R. For the voltage measurement unit, usually the input is a high-impedance input, reaching a level of mega ohms or higher. At this time, the current flowing through R1 is small, and the voltage difference across Rl is very small, so the voltage we measure It is approximately equal to the actual voltage across the resistor.
Another very important factor is the test current. The resistance of some resistors under different test currents will change. The universal expression does not meet such requirements, while the milliohm meter and micro-ohmmeter can select the corresponding constant test current. .
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